Holey optical fibers have microscopic holes or voids for guiding light. In holey fibers, the core is solid (e.g. SiO2) and is surrounded by an array of holes containing inert gas or air.
The light guided in the optical fiber may be confined to the central core region by one of two basic mechanisms. In the first mechanism, light is confined to the central core region by a refractive index difference between the core and cladding material. In conventional solid glass fibers, the refractive index difference is produced by dopants in either the core or cladding material in order to raise or lower the refractive indices of these regions. In general it is desired for the core region to have a higher refractive index than the cladding region. This can either be accomplished by doping materials such as germanium or similar elements in the core to raise the index or doping fluorine or similar in the cladding region to lower the refractive index. The index of the cladding region can also be lowered by introducing porosity in that region. The microscopic holes have a much lower refractive index compared to the solid core, so light is confined to the core. In the second type of confinement mechanism, the size and spacing of the holes is controlled in a very uniform and well defined pattern such that a photonic band gap is produced. The holes must be periodically spaced and carefully arranged and maintained in the fiber to achieve the photonic band gap effects. These fibers are often referred to as photonic crystal fibers owing to their period arrangement of air holes in the fiber. The microscopic holes provide unusual optical properties such as single-mode operation over a wide wavelength range, low zero-dispersion wavelength, and highly controllable birefringence. As a result, holey optical fibers are expected to have a wide range of applications in optical sensors and telecommunications.
Holey optical fibers are conventionally manufactured by stacking an array of hollow silica tubes to form a preform. The tubes are carefully arranged to control the spacing between them and to ensure the crystalline arrangement. The preform is then heated and drawn into fibers as known in the art. The tubes generally experience a uniform scale reduction during drawing so that the tubes create the microscopic holes in the fiber.
One of the drawbacks of the conventional method for making holey optical fibers is the complexity of assembling the stack of tubes. Also, the tube-stacking method cannot be used to produce fibers with random arrays of holes.